1. Field of the Invention
The present invention relates to a height detection apparatus for detecting height information (irregularity information) of a subject, which is measured from a reference surface (thickness information) by using an optical unit.
In particular, the present invention relates to a height detection apparatus used for an image forming apparatus employing an electrophotographic process, which is capable of detecting height information of toner attached on an image bearing member with high precision through application of observation principles of a confocal microscope.
2. Description of the Related Art
Conventionally, there has been known a height detection apparatus for detecting height information of a subject measured from a reference surface by using an optical unit. Such a height detection apparatus is widely used for an optical microscope (confocal microscope), a device for detecting a flaw on a sample surface, and the like.
As the height detection apparatus, there is known a device for detecting a position of a subject surface, that is, detecting height information of a sample surface by using a so-called confocal optical system or another optical system similar thereto (Japanese Patent Application Laid-Open No. 2001-318302).
In the height detection apparatus using the confocal optical system, light beams emitted from a light source unit are condensed on the sample surface, and the light beams reflected from the sample surface are caused to form an image again at a detection unit or in the vicinity thereof. Then, based on a variation in imaging state, information on a height position of the sample surface is detected.
FIG. 6 is a main part schematic diagram of a configuration of a height detection apparatus for detecting height information of a subject by using a confocal optical system.
In FIG. 6, light beams emitted from a light source unit 601 are converted into parallel light beams by a collimator lens 602. After that, the parallel light beams pass through a half mirror 603, and are then condensed on a subject surface 608 by a condensing lens 604.
The light beams reflected from the irradiated subject surface 608 pass through the condensing lens 604 again to be condensed. Then, the light beams are reflected by the half mirror 603. After that, the light beams are condensed at a pinhole slit 606 or in the vicinity thereof by an imaging lens 605.
The light flux that has passed through the pinhole slit 606 is detected for an optical intensity thereof by an optical detector 607 disposed downstream (in a passing direction) of the light flux. Based on the optical intensity detected by the optical detector 607, the height information of the subject surface 608 is detected.
The condensing lens 604 and the imaging lens 605 are disposed in a manner that, when the subject surface 608 is located at the focal position of the condensing lens 604, light reflected from the subject surface 608 forms an image at the pinhole slit 606, enabling the highest optical intensity to be detected at the optical detector 607.
Specifically, as the subject surface 608 becomes further displaced from the focal position of the condensing lens 604 in an optical axis direction thereof, the optical intensity to be detected by the optical detector 607 becomes smaller. Owing to this configuration, a height position of the subject surface 608 in the optical axis direction is detected.
In the height detection apparatus employing the confocal optical system, the height information of the subject surface 608 is detected using the configuration described above.
With the conventional height detection apparatus employing the confocal optical system, there occurs a case where surface height of the subject surface may not be detected accurately because the amount of reflected light detected by the optical detector varies due to variations in reflected light amount caused by the physical properties or roughness of the subject surface.
Further, with the method of only detecting how much intensity the light passing through the pinhole slit has, in principle, it may not be detected whether the subject surface is located in front of the focal point of the condensing lens or behind the focal point.
Further, when the position of the subject surface is displaced significantly from the focal point of the condensing lens, the intensity of the light passing through the pinhole slit decreases proportionately with the square of the displacement in height of the subject surface. As a result, detection precision for the position of the subject surface decreases extremely.
In a focus detecting device disclosed in Japanese Patent Application Laid-Open No. 2001-318302, light fluxes emitted from a linear light source (one-dimensional light source) having multiple point light sources arranged in one direction are condensed in line on a subject surface. Similarly, an imaging lens provided on an optical detector side causes the light reflected from the subject surface to form an image in line. Then, by using a one-dimensional optical sensor tilted by a minute angle with respect to an imaging surface about an axis perpendicular to the optical axis and the line image, the light reflected from the subject is detected.
However, with this method, when there are variations in height along the focal line of the subject surface, light beams reflected on the subject surface of multiple heights enter the optical detector. Accordingly, precise measurement is difficult to achieve with regard to the height information of the subject surface in a narrow area.